Internal-combustion engines commonly provide motive power for operating electrical generators, motor vehicles, and many other uses. Most such engines are bad polluters because they operate on fossil fuels, which contain a wide variety of components incapable of being fully combusted together within their brief residence in an engine.
Combustion effluents of fossil fuels, such as coal, petroleum (including diesel oil, kerosene, jet fuel, and gasoline), and even natural gas, contain--in addition to customary carbon dioxide and water--undesirable hydrocarbon fragments and derivatives, often in particulate form, carbon monoxide, and gaseous oxides of nitrogen and/or of sulfur transformable to noxious droplet form (acid rain).
Hydrogen was long-considered an ideal fuel because convertible completely to water via air combustion. See century-old Eldridge U.S. Pat. No. 603,058 for Electrical Retort wherein an electric arc flashed water to steam, then pyrolyzed it and carbon electrodes in a high-pressure reaction vessel, in order to recover hydrogen.
Yet hydrogen is unsatisfactory as an internal-combustion engine fuel, because the high temperature and the rapidity of its combustion foster pre-ignition or flashback, which is greatly harmful to engine operation and structure. Also flashback is conducive to an increase of harmful nitrogen oxides (aptly: "NOx") in the atmosphere.
An instructive reference is HYDROGEN STORAGE AND UTILIZATION IN TRANSPORTATION VEHICLES--SUMMARY, United States Department of Energy, Alternative Fuels Utilization Program, Office of Transportation Systems (1988). Despite the intervening decade of research and development, no vehicle running on hydrogen as its main or sole fuel is yet commercial, notwithstanding much experimentation on fuel cell technology, which is fundamentally electrolytic and slow-generating.
Other commercial fuels and their uses, such as acetylene burning in a cutting or welding torch, have similar drawbacks. Acetylene also requires a higher concentration of oxygen than is present in air to enable the desired high temperature and facility of operation.